When a gate voltage of an n-type FET exceeds a threshold voltage (Vth), the n-type FET becomes ON state and the resistance between its source and drain is lowered. In a normal FET, Vth is a constant value regardless of whether it is in ON state or OFF state. There has been proposed an n-type FET having a function of reducing Vth when turned on, and increasing Vth again when turned off. This function is called Vth self-adjusting function.
For example, one prior-art discloses an. SRAM utilizing an FET having the Vth self-adjusting function by using a ferroelectric film (hereinafter referred to as Vth self-adjusting FET). By forming an SRAM using the Vth self-adjusting FET, the SRAM can be improved both in data retention characteristics and read characteristics. The disclosed Vth self-adjusting FET has a gate stack formed of a buffer film made of Hf—Al—O on a silicon substrate, an SrBi2Ta2O9 (SBT) ferroelectric film on the buffer film, and a Pt metal electrode on the SBT film.
As other conventional techniques, there are disclosed a technique for improving Ioff performance by a nonvolatile programmable switch using a nonvolatile memory Tr, and a high Vth transistor using a power switch FET, and a technique for improving the capture rate and retention rate of high-energy electrons by increasing the film thickness of a charge trap film.